Method and apparatus for correcting medical imaging data

ABSTRACT

In a method and apparatus for correcting image data from a medical imaging scan of a subject, into which subject a specified amount of an imaging substance has been introduced, a region of the image data, containing an anomalous proportion of the imaging substance introduced, is identified. For the identified region a regional value of a variable in the image data associated with the imaging substance is determined. The regional value is used to determine the proportion of the substance in the region, and the proportion is subtracted from the specified amount of the imaging substance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to methods and apparatus for correcting forimage data from a medical imaging scan of a subject, into which subjecta specified amount of an imaging substance has been introduced, inparticular to dose infiltration in PET imaging.

2. Description of the Prior Art

In the medical imaging field, several imaging schemes are known. Forexample PET (Positron Emission Tomography) is a method for imaging asubject in 3D using an injected radio-active substance which isprocessed in the body, typically resulting in an image indicating one ormore biological functions. Other such functional imaging modalities areknown, such as SPECT.

In such functional images, many important pathologies and anatomicalstructures appear as very high (or low) intensities. For example, atumour in an FDG-PET image will often appear as a bright region.

When a PET radiotracer is injected into a patient, dose infiltration mayoccur when not all of the radiotracer is injected into the vein. Themaximal intensity projection (100) shown in FIG. 1 illustrates a casewith dose infiltration at the injection site (102). The infiltration canbe seen as highly increased uptake in the region (104) around theinjection site. In this case the maximum SUV at the site of infiltrationis more than 4000.

The potential impacts of this on the clinical workflow are as follows:

1. Axial slices in which an infiltration site is present suffer fromreduced intensity granularity due to rescale slope and intercept (RSI)encoding in DICOM, in addition to other image artifacts that affect theclinical readability of those slices.

2. Standardized Uptake Values (SUVs) for the entire patient are nolonger reliable as the infiltrated dose is not able to circulate aroundthe body, so only a fraction of the injected dose is available foruptake by tissue.

Regarding the first of the potential clinical impacts listed above, ifthe dose infiltration is detected before the patient leaves imagingfacility, the bed position affected by the infiltration site can berescanned with the infiltration site moved outside the field of view(FOV).

Rescanning the affected bed position will not, however, address thesecond problem of unreliable SUVs. This can only be addressed byrepeating the entire imaging process at a later time, exposing thepatient to additional dose from the radiotracer and the CT scan forattenuation correction.

Dose infiltration also affects other imaging modalities (such as MR, orultrasound) in which imaging substances are introduced to the subject,where not all of the imaging substance is properly introduced, either byinjection or insertion, so that the initial amount of the imagingsubstance is reduced by an unknown proportion.

SUMMARY OF THE INVENTION

An object of the present invention is to address the above-describedproblems and to provide improvements upon the known devices and methods.

In general terms, an embodiment of a method according to the inventionfor correcting image data supplied to a computerized processor from amedical imaging scan of a subject, into which subject a specified amountof an imaging substance has been introduced, includes, in the processor,identifying a region of the image data containing an anomalousproportion of the imaging substance introduced, determining for theidentified region a regional value of a variable in the image dataassociated with the imaging substance, using the regional value todetermine the proportion of the substance in the region, subtracting theproportion from the specified amount of the imaging substance to obtaina corrected amount of the imaging substance, and making an indication ofthe corrected amount available from the processor.

This allows a simple of correction of the amount of the imagingsubstance which is used for later calculation, by the amount lost byanomalous introduction into the subject, for example by doseinfiltration into tissue surrounding a vein.

Preferably, the imaging substance is a pharmaceutical tracer. Morepreferably, the variable is activity of the tracer. Still morepreferably, the step of determining comprises measuring an SUV value forthe region.

The method can further include determining the mean activityconcentration in the identified region, and a volume, to determine atotal activity in the identified region.

Preferably, the steps of using and subtracting include subtracting thetotal activity from a decay-corrected dose introduced.

In an embodiment, the identified region is an infiltration site of thesubject.

Suitably, the anomalous proportion is a part of a dose not injected intoa vein.

In one embodiment, the step of identifying is registering (entering) auser selection of the region.

In another embodiment, the step of identifying is detecting a region inthe image data having activity over a given threshold.

Preferably, the region detected includes one or more voxels having avalue of SUV over the threshold.

More preferably, the method further includes segmenting the detectedregion, for example using an isocontour at a predetermined percentage ofa maximum activity value for the region.

In one embodiment, the method further includes initially comparing theimage data to an anatomical atlas, and identifying, as candidates forselection, regions of the image data corresponding to regions in theatlas marked as possible sites of anomalous introduction of thesubstance.

The invention also encompasses an apparatus for correcting for imagedata from a medical imaging scan of a subject, into which subject aspecified amount of an imaging substance has been introduced. Theapparatus includes a processor configured to identify a region of theimage data containing an anomalous proportion of the imaging substanceintroduced, determine for the identified region a regional value of avariable in the image data associated with the imaging substance, usethe regional value to determine the proportion of the substance in theregion, and subtract the proportion from the specified amount of theimaging substance; and a monitoring device configured to indicate thecorrected amount of the imaging substance to a user.

The present invention also encompasses a non-transitory,computer-readable data storage medium encoded with programminginstructions that, when the storage medium is loaded into a computerizedprocessor, cause the processor to implement one or more of theembodiments of the inventive method described above.

The above aspects and embodiments may be combined to provide furtheraspects and embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a dose infiltration site in a MIP according to anembodiment of the invention.

FIG. 2 shows flowcharts illustrating steps according to embodiments ofthe invention.

FIG. 3 illustrates an apparatus according to an embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

When the following terms are used herein, the accompanying definitionsare applicable:

-   -   PET—Positron Emission Tomography    -   SUV—Standardised Uptake Value    -   FDG—F-18 fluorodeoxyglucose, a PET radiotracer    -   MIP—Maximum Intensity Projection (or Minimum Intensity        Projection, usually denoted MinIP)    -   MRI—Magnetic Resonance Imaging    -   ROI/VOI—Region/volume of interest.    -   CT—Computed Tomography    -   DICOM—Digital Imaging and Communications in Medicine    -   FOV—Field Of View    -   RSI—Rescale Slope and Intercept

The invention provides a method for correcting for inadvertentintroduction of some imaging substance at a site in the subject, byassessing the region containing the substance (inadvertently introduced)and comparing this with the initial amount of the substance introduced.

Embodiments of the invention provide features such as

-   -   automatically detecting non-circulating radiotracer        (infiltration) at the injection site and correcting        injected-dose-normalised measures of radiotracer uptake    -   the presence of dose infiltration is detected based upon the        presence of abnormally-high activity concentration    -   the activity trapped at the infiltration site is classified and        measured using an isocontour delineated sub-volume    -   the trapped activity measured is subtracted from the        (decay-corrected) total injected activity for re-computation of        correcting injected-dose-normalised measures of radiotracer        uptake    -   detection of dose infiltration is refined using anatomical        information, for example, registration to an anatomical atlas to        restrict search to the arms.

An embodiment of the invention allows the system to provide correctedSUVs that account for non-circulating tracer trapped at an infiltrationsite. In this embodiment, it does this as follows, with reference toFIG. 2:

1. The patient data is loaded (202), and the user manually identifiesand delineates the infiltration site in the image reviewing software.Alternatively, the infiltration site could be automatically identifiedby the system by detecting (204) any voxels with an SUV over a giventhreshold (e.g., 500) then segmenting (206) the surrounding region usingan isocontour (e.g., at some fixed percentage of SUVmax).

2. The total activity (208) in the delineated infiltration site iscalculated (i.e., mean activity concentration (Bq.ml-1)×volume (ml)).

3. The infiltration activity is then subtracted (210) from thedecay-corrected injected dose value used in the computation of SUV:

${SUV} = {\frac{C}{D - I}*W}$where C is the activity concentration measured in a voxel or region ofinterest (Bq.ml-1), D is the dose injected (Bq) following decaycorrection to image start time, I is the measured infiltration activity(Bq), and W is the patient weight (g).

The second flow chart in FIG. 2 gives an example, which is based on thesubject image data record captured and illustrated in FIG. 1. The loadedpatient data (212) returns a maximum voxel SUV of 4062 (214), which notonly indicates an anomaly at the site 102, but indicates the location ofthe site.

The isocontour segmented has 530 voxels surrounding the site (216), andtotal activity is then calculated and subtracted to find the correcteddose (218, 220).

When using the automated method for finding the infiltration site, it isnotable that by the time (at the time point in the data set) theinfiltration is clear in the image data, the tracer injected which hasentered the vein correctly, will have long passed away from theinjection site into the subject, and therefore any remaining activity inthis region should be that due to the does infiltration. A correctionfor any background activity can be performed if necessary.

The automatic detection of the infiltration site can be refined using ananatomical atlas, e.g., by restricting the search to potential injectionsites such as the arms.

If the infiltration activity is very high, the imaging system mayunderestimate the true counts measured (due to saturation/dead timeetc.). In such cases, an additional correction for this may be appliedwhere available.

The infiltration site could be detected at image acquisition (by anunusually high number of counts in the sinogram bins corresponding to aparticular region. Auto correction could then be performed at this pointby providing a corrected injected dose in the DICOM header for SUVcomputation (as described above). Furthermore, the activity stored inthe reconstructed image voxels at the infiltration site could be reducedto a less extreme value so as to improve the granularity of intensityvalues in those image slices following RSI encoding.

In other embodiments of the invention, the assessment of the anomalousintroduction of the imaging substance can be applied to other imagingmodalities using imaging substances, such as MR, or ultrasound. Forexample, in the case of MR, the use of contrast agents can be affectedby infiltration or leakage of the agent into unintended areas. Insimilar fashion to the above method, the region of the imaging data ofthe subject affected can be obtained, and the amount of agent in theregion identified, and subtracted from the initial amount introduced.For example, the additional contrast in the region selected could bemeasured, either directly or in comparison with a background, and thiscompared to the contrast expected for a standard amount of the agent,and this used to calculate the amount of agent in the affected area.

Referring to FIG. 3, the above embodiments of the invention may beconveniently realized as a computer system suitably programmed withinstructions for carrying out the steps of the methods according to theinvention.

For example, a central processing unit 304 receives data representativeof medical scans via a port 305 which could be a reader for portabledata storage media (e.g. CD-ROM); a direct link with apparatus such as amedical scanner (not shown) or a connection to a network. For example,in an embodiment, the processor performs such steps as: identifying aregion of the image data containing an anomalous proportion of theimaging substance introduced; determining for the identified region aregional value of a variable in the image data associated with theimaging substance; using the regional value to determine the proportionof the substance in the region; and subtracting the proportion from thespecified amount of the imaging substance.

Software applications loaded on memory 306 are executed to process theimage data in random access memory 307.

A Man—Machine interface 308 typically includes a keyboard/mouse/screencombination (which allows user input such as initiation of applications)and a screen on which the results of executing the applications aredisplayed.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventor to embody within the patentwarranted hereon all changes and modifications as reasonably andproperly come within the scope of his contribution to the art.

I claim as my invention:
 1. A method of correcting for image datasupplied to a computerized processor from a medical imaging scan of asubject, into which subject a specified amount of a pharmaceuticaltracer has been introduced, comprising, in the processor, the steps of:accessing anatomical data in an anatomical atlas and comparing the imagedata to said anatomical data, and identifying a set of candidate regionsof the image data that correspond respectively to regions in theanatomical atlas marked as likely sites of introduction of thepharmaceutical tracer into the subject; detecting a region of the imagedata that represents activity produced by said pharmaceutical tracerthat is above a predetermined threshold, and confining detecting saidregion of the image to only detecting said activity that is above saidpredetermined threshold in said candidate regions; determining for thedetected region a regional value in the image data associated with thepharmaceutical tracer; using the regional value to determine an amountof the pharmaceutical tracer in the region; subtracting said amount fromthe specified amount of the pharmaceutical tracer to obtain a correctedamount of the pharmaceutical tracer; and making an indication of thecorrected amount available from the processor.
 2. A method according toclaim 1, wherein the variable is said activity produced by thepharmaceutical tracer.
 3. A method according to claim 2, comprisingdetermining said region value by measuring an SUV value for the region.4. A method according to claim 1, further comprising determining anaverage concentration of said activity in the identified region, and avolume of said region, and determining a total activity in theidentified region as a product of said average concentration and saidvolume.
 5. A method according to claim 4, wherein the steps of using andsubtracting comprise: subtracting the total activity from adecay-corrected dose introduced into the subject.
 6. A method accordingto claim 1, wherein the identified region is an infiltration site of thesubject.
 7. A method according to claim 1, comprising detecting theregion as comprising one or more voxels having a value of SUV over thethreshold.
 8. A method according to claim 1, further comprisingsegmenting the detected region from said image data.
 9. A methodaccording to claim 8, comprising segmenting the region using anisocontour at a predetermined percentage of a maximum of said activityfor the region.
 10. An apparatus for correcting image data from amedical imaging scan of a subject, into which subject a specified amountof pharmaceutical tracer has been introduced, the apparatus comprising:an anatomical atlas having anatomical data stored therein; a processorconfigured to access said anatomical data in said anatomical atlas andto compare the image data to said anatomical data, and to identify a setof candidate regions of the image data that correspond respectively toregions in the anatomical atlas marked as likely sites of introductionof the pharmaceutical tracer into the subject; a processor configured todetect a region of the image data that represents activity produced bysaid pharmaceutical tracer that is above a predetermined threshold, andto confine detecting said region of the image to only detecting saidactivity that is above said predetermined threshold in said candidateregions, and to determine for the detected region a regional value inthe image data associated with the pharmaceutical tracer, and to use theregional value to determine an amount of the pharmaceutical tracer inthe region, and to subtract said amount from the specified amount of thepharmaceutical tracer to obtain a corrected amount of the pharmaceuticaltracer; and a monitoring device at which the processor is configured toindicate the corrected amount of the pharmaceutical tracer to a user.11. A non-transitory, computer-readable data storage medium encoded withprogramming instructions, said data storage medium being loaded into acomputerized processor that is supplied with image data from a medicalimaging scan of a subject, into which subject a specified amount ofpharmaceutical tracer has been introduced, said programming instructionscausing said computerized processor to: access anatomical data in ananatomical atlas and compare the image data to said anatomical data, andidentify a set of candidate regions of the image data that correspondrespectively to regions in the anatomical atlas marked as likely sitesof introduction of the pharmaceutical tracer into the subject; detect aregion of the image data that represents activity produced by saidpharmaceutical tracer that is above a predetermined threshold andconfine detecting said region of the image to only detecting saidactivity that is above said predetermined threshold in said candidateregions; determine for the detected region a regional value in the imagedata associated with the pharmaceutical tracer; use the regional valueto determine an amount of the pharmaceutical tracer in the region;subtract said amount from the specified amount of the pharmaceuticaltracer to obtain a corrected amount of the pharmaceutical tracer; andmake an indication of the corrected amount available from the processor.